The present invention relates to an external combustion engine which combines some of the advantages of gas turbines and piston engines by means of the use of a valve-less free-piston combustor. The use of gas turbines in conjunction with that of piston engines is well known and widely used in the form of turbo-chargers. The present invention engine differs from such turbo-charged piston engines in that: (1) the combusted gas is expanded in the turbine to produce the engine shaft power, (2) no shaft power is directly extracted from the energy generated by the free-piston member, and (3) the turbine admission pressure may be appreciably higher than that of the compressed air entering the combustor. The latter characteristic also differentiates it from conventional gas turbines.
External combustion engines (EC engines as opposed to IC engines) have been the object of a few patents, in the recent past, and been granted to the present inventor. These are mentioned here as pertinent references hereto:
1. U.S. Pat. No. 4,399,654 issued Aug. 23, 1983. Pending in Japan: Application No. 024912/83 filed Feb. 19, 1982; and in the European Patent Office: Application No. 83300628.1 filed 08/02/83. PA0 2. U.S. Pat. No. 4,561,252 issued Dec. 31, 1985. PA0 3. U.S. Pat. No. 4,653,273 issued Mar. 31, 1987. PA0 3. U.S. Pat. No. 4,653,274 issued Mar. 31, 1987. PA0 4. U.S. Pat. No. 4,662,177 issued May 5, 1987. PA0 5. U.S. Pat. No. 4,665,703 issued May 19, 1987. PA0 6. U.S. Pat. No. 4,672,813 issued June 16, 1987. PA0 7. U.S. Pat. No. 4,702,205 issued Oct. 27, 1987. PA0 8. U.S. patent application Ser. No. 893,701 filed 08/06/86, now abandoned, entitled "External Combustion Rotary Engine".
The free-piston combustor construction has evolved into a configuration in which piston friction is eliminated by means of a plurality of compressed air cushions isolating the oscillating piston from the sleeve. The pulsed-combustion frequency is one tenth of that which characterizes conventional piston engines. Considerable more time is thus available to burn less volatile fuels more effectively. Motors are of the vane or rotary types. Compressed-air cushioning of moving parts is widely used so as to eliminate friction and the need of lubrication. However, some of the congenital disadvantages of the motor types just mentioned such as: need of valves in rotary motors, vane vibrations in vane motors and rotational speed (rpm) limitations for both, point to the direction of gas turbines as ideal motors. Because of their much higher rpm characteristics, continuous air/gas flow feature, absence of contact between moving parts, etc., gas turbines yield a very high ratio of shaft power to both volume and weight. However, they were limited until recently by some inherent drawbacks such as: relatively low efficiency (high fuel consumption), need of a high-ratio down-gearing and high cost.
Thermodynamic efficiency can be improved appreciably by way of higher turbine operating temperatures which the advent of ceramic materials has made possible. The use of a free-piston combustor permits to lower the maximum rpm of the turbine, while enabling both the overall compression ratio of the engine and its peak operating temperatures to increase. The concomitant result is an appreciable increase in engine overall efficiency.
In view of this background, it is therefore desirable to combine the advantages mentioned above while minimizing the influence of or eliminating the disadvantages also mentioned above for each type of motors. This combination of various technologies results in a new type of hybrid engine with considerable promises.